The present invention is directed to a blast heating apparatus, particularly for blast furnaces, of the type including an inner metal shell, for example a shell formed of sheet iron, for example by welding together plural sheet iron courses, having positioned therein a refractory fireproof lining.
It is known that modern blast heating apparatus, which operate with relatively high dome temperatures of up to 1600.degree. C. and hot blast pressures of up to 4 bar, suffer from intercrystalline tension crack corrosion problems. In many cases, the sheet iron shell surrounding the fireproof lining is damaged prematurely to such an extent that for reasons of operational safety a lower dome temperature has to be used to prevent damage and/or prevent shut-down of the blast heating apparatus. This lower operating temperature reduces the efficiency of the entire installation. It generally is considered that the cause of such intercrystalline tension crack corrosion is the occurrence of nitrates under tensile stress, which lead to the formation of cracks in the surface of the metal shell.
West German DE-OS No. 27 42 109 deals with the problem of tension crack corrosion and indicates that such phenomenon may be avoided by the provision of a protective shell which is welded to the inside wall of the main or first shell and which is elastically deformable under thermal and mechanical stresses. Preferably, this protective shell is formed of a high alloy special steel material. Due to the interior installation of the protective shell with regard to the main or first shell, this solution primarily is intended for new constructions, and this system can be employed in existing blast heating apparatus only at great costs.